The invention relates to cellular communication systems, and more particularly to restoration of user equipment control in the presence of communication link failure between a network node that provides control of packet switched connections and a network node that provides control of circuit switched connections.
Various aspects of embodiments are described with reference to one or more standards issued by the Third Generation Partnership Project (3GPP), and the terminology used in those standards is often used herein. This is done for the purpose of facilitating an understanding of the described embodiments because the standardized terminology and concepts are well known to those skilled in the art. However, the reference to particular standards as well as the use of such terminology and examples is not intended to mean or suggest that the various described aspects are applicable only in environments that conform to the mentioned standards. To the contrary, the issues described herein as well as the disclosed solutions and features can arise in technological environments that do not conform to the mentioned standards, and it is fully intended that the described embodiments are intended only as examples and are not limited to the particular embodiments or standards disclosed herein.
As illustrated by FIG. 1, a User Equipment (UE) 101 may interact in the digital realm (i.e., via the communication of data packets) with the Evolved Packet Switching system (EPS) 100 using the radio access mechanisms provided by the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) 103. The interface between the UE and E-UTRAN is denoted “LTE-Uu”. This latest equipment provides communication services to a UE 101 (both voice and other information) by means of Packet Switched (PS) technology. UE-related control signaling is handled by a Mobility Management Entity (MME) 105 with support of subscription information provided by the Home Subscriber Server (HSS) 107. The MME 105 communicates with E-UTRAN 103 via an S1-MME interface. The MME 105 interacts with the HSS 107 via an S6A interface. The MME can also interact with one or more other MMEs by mean of an S10 interface.
User payload is handled by the Serving Gateway (S-GW) 109 and the Packet Data Network (PDN) Gateway (P-GW) 111. The S-GW 109 is coupled to E-UTRAN via an S1-U interface, and the S-GW 109 is coupled to the P-GW 111 via an S5 interface. The P-GW 111 may interact, via a Gx interface, with a Policy and Charging Rules Function (PCRF) 113.
An operator's Internet Protocol (IP) Services 115 (e.g., (Internet Protocol) Multimedia Subsystem (IMS), etc.) can be reached by the P-GW 111 via an SGi interface, and the PCRF 113 can interact with the operator's IP Services 115 via an Rx interface.
The system also provides for interaction with legacy system technology. For example, the MME 105 can interact with a Serving GPRS (General Packet Radio Service) Support Node (SGSN) 117 via an S3 interface. To support Circuit Switched (CS) communication links, a Mobile Switching Center (MSC) 119 is provided, and this is able to communicate with the MME 105 by means of two different interfaces: an SGs interface and an Sv interface.
Because deploying a new type of system is a gradual process, mechanisms are put in place that allow the new (e.g., PS-based) system to coexist with legacy (e.g., CS-based) systems that have already been put into place, and it is useful (and sometimes necessary) to allow a UE's service to switch from one type of service support to another, based upon its movement and the type of service support that is available in any particular area. Present embodiments, therefore, permit a UE to obtain service not only from the latest packet switched equipment, but also from legacy systems that employ CS technology. Registration for the CS domain is performed by the MME 105 and a (legacy system) Mobile Switching Center (MSC) 119 by means of interaction at the illustrated SGs interface. As is known in the art, Single Radio Voice Call Continuity (SRVCC) is a process/technology by which an ongoing voice call that is presently being handled by a network's Voice over IP (VoIP)/IP (Internet Protocol) Multimedia Subsystem (IMS) in the data packet switched domain can be transitioned to a legacy system's circuit switched domain. The SRVCC feature is transacted over the illustrated Sv interface.
As shown in FIG. 2a, the 3GPP network may be organized using multiple MMEs serving the same tracking areas of a Long Term Evolution (LTE) radio network. The organization of multiple MMEs is referred to as an “MME Pool”.
In the example of FIG. 2a, a first MME Pool 201 comprises MMEs 1 and 2 and serves the Evolved Node B (eNB) service areas 1 and 2 identified as PS pool-area 1. A second MME Pool 203 comprises MMEs 3, 4, and 5, and serves the eNB service areas 3, 4, 5, and 6 identified as PS pool-area 2. Although not shown in this example, it will be noted that one or more of the eNBs could be in more than one PS pool-area (e.g., Area 3 could be in both PS pool-area 1 and PS pool-area 2).
As shown in FIG. 2b, the 3GPP network may also be organized using multiple SGSNs serving the same routing areas of a GERAN (GSM (Global System for Mobile communication) EDGE (Enhanced Data for GSM Evolution) Radio Access Network)/UTRAN radio network. The organization of multiple SGSNs is referred to as an “SGSN Pool” Likewise, it is possible for the 3GPP network to include an organization of multiple MSCs serving the same location areas of a GERAN/UTRAN radio network, with that organization being referred to as an “MSC pool”.
In the example of FIG. 2b, a first MSC Pool 205 comprises MSCs 1, 2, and 3, and serves the RAN node service areas 1, 2, 5, and 6 identified as CS pool-area 1. A second MSC Pool 207 comprises MSCs 4, 5, and 6, and serves the RAN node service areas 2, 3, 6, and 7 identified as CS pool-area 2. It will be observed that two of the RAN node service areas, namely areas 2 and 6, are in both CS pool-area 1 and CS pool-area 2.
FIG. 2b further illustrates a first SGSN Pool 209 that comprises SGSNs 1 and 2, and serves the RAN node service areas 1 and 5 identified as PS pool-area 1. A second SGSN Pool 211 comprises SGSNs 3, 4, and 5, and serves the RAN node service areas 2, 3, 6, and 7 identified as PS pool-area 2.
FIG. 2b also shows a single MSC 213 serving RAN service areas 4 and 8; and also shows a single SGSN 215 serving RAN service areas 4 and 8.
MME Triggered Service Restoration Procedure
There are two different failure scenarios applicable to SGs interface that may trigger the MME to perform a re-registration of a UE in the CS domain, either in the previously serving MSC or in a new MSC. The case involving a new MSC requires that the MSC pool be deployed. The two cases are                Path failure: a break in the signaling control path between the MME and the MSC. This is detected by broken communication. There may be any of a number of reasons for the break, such as one of the node peers being no longer operational, the transport network being no longer operation, and the like.        The MSC or MME being restarted: This is detected by a restart indication in communication between the nodes once both nodes are operational again.        
When the MME receives an indication from the serving MSC/VLR that a VLR restart has occurred, or if the VLR is no longer in service if there are no more Stream Control Transmission Protocol (SCTP) associations in service with that VLR for a given period of time, then the MME initiates an SGs restoration procedure as follows:                1) Upon receipt of a combined Tracking Area (TA) update request or periodic Tracking Area Update request from a UE that is attached to both EPS and non-EPS services, the MME may request the UE to re-attach to non-EPS services or the UE may alternatively immediately perform the Location Update (LU) for non-EPS services procedure towards the Visitor Location Register (VLR).        2) Upon reception of a Combined TA/LA (update or periodic Tracking Area Update from a UE that is attached for non-EPS service), the MME may either:                    a. request the UE to re-attach to non-EPS services and then select an alternative available VLR to serve the UE for CS services during the subsequent combined TA/LA update procedure;            b. or immediately perform the Location Update for non-EPS services procedure towards an alternative available VLR.                        3) Upon reception of an Uplink Non-Access Stratum (NAS) Transport message from a UE that is attached for non-EPS service, if the Visitor Location Register (VLR) serving the UE is no longer in service, the MME may request the UE to re-attach to non-EPS services and then select an alternative available VLR to serve the UE for MO Short Message Service (SMS) and other CS services during the subsequent combined TA/LA update procedure. See 3GPP TS 29.118 for the details. Although it is not specified in the current standard, this procedure is also a functionally viable approach for handling instances in which the UE is already in the ECM-CONNECTED state.        
For the Sv interface, since there is no registration procedure towards the MSC for a given UE, which is unlike for SGs interface, where the SGs association must be established before CS fallback procedures can be continued, the MME will contact an MSC only when an SRVCC PS to CS handover is required; that is, when a UE is moving toward a 2G/3G coverage area and leaving an LTE coverage area, where the ongoing voice call should be transferred from the PS domain to the CS domain. So, when SRVCC functionality has been invoked and a path failure on the Sv interface has been detected when trying to make an SRVCC PS to CS Request towards the selected MSC, then the MME tries to perform the SRVCC PS to CS procedure to any other available MSC using the Sv interface. This requires of course that there be more than one MSC serving Sv and the UEs location (i.e., the MSC pool must be deployed). But, when no MSC is available to the MME, the restoration procedure initiated by the MME must be triggered; otherwise the voice call will be terminated due to the UE having left the LTE coverage area.
MSC Initiated SGs Restoration (See 3GPP TS 23.007, Chapter 26 for Further Details)
When the VLR has to page the UE for a Mobile Terminated (MT) CS service (e.g., upon receipt of an incoming CS call), if the VLR detects that the MME serving the UE is no longer in service, the VLR should send an SGs paging request with a CS restoration indicator to an alternative MME in the same MME pool. The VLR should load-balance the paging requests among the available MMEs in the pool during the restoration procedure in order to avoid unduly burdening one or only a few of the MMEs in the pool.
The VLR may know the set of MMEs pertaining to the same MME pool by local configuration or by checking the MME Group ID within the MME name that MMEs signal to the VLR in the SGsAP-LOCATION-UDATE-REQUEST, SGsAP-RESET-INDICATION or SGsAP-RESET-ACK messages. The MME should send an SGsAP-RESET-INDICATION message to the VLR after restart.
The VLR may detect that an MME is no longer in service if there are no more SCTP associations in service with that MME.
The MME shall accept the SGs paging request and proceed as follows upon receipt of an SGs paging request that includes the CS restoration indicator:                If the International Mobile Subscriber Identity (IMSI) is unknown by the MME, or if the IMSI is known and the UE is marked as EPS Mobility Management (EMM)-DEREGISTERED, the MME shall send the paging request with the location information provided by the VLR, regardless of the value of the “MME-Reset” indicator. If no such location information is provided, the MME may either page the UE in all the tracking areas corresponding to that MME or in the tracking areas served by the MME and by the VLR, or reject the paging request per operator policy. The paging request shall include the IMSI and the CN domain indicator set to “PS” to request the UE to re-attach;        If the IMSI is known by the MME and the UE is considered to be attached to both EPS and non-EPS services or for SMS only (for an SGs paging request with an “SMS indicator”), the MME shall page the UE based on the location information stored in the MME.        
Upon receipt of a paging request including the IMSI and the CN domain indicator set to “PS”, the UE re-attaches to one MME of the pool (the particular one may not necessarily be the MME that initiated the paging procedure towards the UE) and a new SGs association is established with the VLR. This VLR may not be the same VLR that initiated the SGs paging procedure (e.g., if Intra Domain Connection of RAN Nodes to Multiple CN Nodes is deployed for GERAN or UTRAN (see 3GPP TS 23.236)).
If the new SGs association is established towards the same VLR, the VLR should repeat the SGs paging request after the UE has re-attached to non-EPS services. The MT CS service or SMS is then delivered according to normal procedures.
If the new SGs association is established towards a different VLR, the MT CS service may be delivered via the new VLR using Mobile Terminating Roaming Retry or Mobile Terminating Roaming Forwarding (see 3GPP TS 23.018); the on-going MT SMS is retransmitted by the SMS-SC using the existing SMS procedures (SMS alert).
It is evident that, in each of the above-described restoration procedures, the MME must be able to reach an available MSC. However, this may not be possible in all circumstances.
In a scenario in which the MME is unable to reach an MSC, the UE is not served by combined procedures but is instead limited only to Evolved Packet Core (EPC) services only. Further, in such instances SRVCC PS to CS handover will not be possible.
The existing behaviors specified in the current 3GPP specifications are not optimal if the communication error is local to an MME in an MME pool and there are other MMEs in the same pool which do have a connection to suitable MSC(s). In this case, it is not optimal that the UE is denied CS services despite that MSC(s) is/are available to the MME pool when using a different MME.
When an MME suffers from a connectivity outage to all MSCs, i.e. when there no longer is any connectivity at interface SGs, the MME will no longer receive any MT call or serve a Mobile Originated (MO) call for served UEs that are combined attached. Nor will SRVCC be possible.
In case the issue is local to a single MME it may be resolved by having the serving MME force the UE to be moved to a different MME within the MME Pool, for example by executing load rebalancing Tracking Area Update (TAU).
However, it is herein observed that there are some issues with that approach.
If the SGs/Sv connectivity error is common to all MMEs in the MME Pool, the new MME will also fail to perform SGs/Sv signaling. The result would be that the new MME would also deny the UE's combined attachment. Nor will SRVCC be possible. Further, extra signaling will be induced by moving the UE from one MME to another.
The UE would then either attempt to perform an access change to get CS attached in 2G/3G (voice centric), or would alternatively remain in the LTE access and no longer be able to use CS services (data centric). An ongoing Voice over LTE (VoLTE) call may also be dropped when going out of LTE coverage.
If the UE remains in the LTE access, the behavior is at risk of going cyclic with a never ending sequence of inter-MME TAU procedure executions impacting all UEs that make requests to get interface (I/F) SGs-related services.
In case the issue is local to only some MMEs in the MME Pool, the approach would succeed and the UEs would eventually be registered by an MME that is able to signal at the I/F SGs.
However, it would at the same time create a load balancing problem since the data centric UEs that chose to remain being served in the LTE access are moved from an MME that has lost SGs connectivity to an MME that does not have that problem. The result is that all UEs that request Circuit Switched FallBack (CSFB—so-called because this feature enables voice services to be delivered to packet-capable equipment by means of legacy circuit-switched network elements) are concentrated to some MMEs in the pool, thereby making the UE distribution and the resulting node load no longer randomized over the nodes in the pool.
When the situation normalizes it would be necessary to redistribute UE registrations over the pool to get an even load distribution. But since the load distribution is no longer randomized, this cannot be easily performed by existing load distribution mechanisms.
In view of the above-described issues, it is desired to provide technology (e.g., methods, apparatuses, processor-readable storage medium, etc.) that are capable of providing a backup mechanism for establishing a reconnection between, on the one hand, an entity such as an MME and, on the other hand, an entity such as an MSC when an attempted primary reconnection mechanism experiences a problem.